I hope I'm not suggesting something silly, but would it be possible to perform a test with engine running at 2200 RPM with propeller removed? Depending on the noise still being present or not, that could provide some useful clues.
On the other hand, I suppose removing a propeller is not a trivial operation, at least regarding security aspects. And running an engine without its propeller affixed may just prove impractical...
Mechanics ? What do you say ?